At the center of the Sun, where its density is 1.5 × 105 kg m-3, thermonuclear reactions (nuclear fusion) convert hydrogen into helium. 3.9 × 1045 atoms undergo nuclear reactions there every second. This releases energy which escapes from the surface of the Sun as light. Physicists are able to replicate thermonuclear reactions with hydrogen bombs. Sustained nuclear fusion on earth for electricity generation may be possible in the future, with nuclear fusion reactors.

All matter in the Sun is in the form of plasma due to its extreme temperature. This makes it possible for the sun to rotate faster at its equator than it does at higher latitudes, since the Sun is not a solid body. The differential rotation of the Sun's latitudes causes its magnetic field lines to become twisted together over time, causing magnetic field loops to erupt from the sun's surface and trigger the formation of the Sun's dramatic sunspots and solar prominences[?].

For some time it was thought that the number of neutrinos produced by the nuclear reaction in the Sun was only one third of the number predicted by theory, a result that was termed the solar neutrino problem. When it was recently found that neutrinos had mass, and could therefore transform into harder-to-detect varieties of neutrinos while en route from the Sun to Earth, measurement and theory were reconciled.